18717249721 has 3 divisors (see below), whose sum is σ = 18717386533. Its totient is φ = 18717112910.

The previous prime is 18717249667. The next prime is 18717249763. The reversal of 18717249721 is 12794271781.

It is a happy number.

The square root of 18717249721 is 136811.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a de Polignac number, because none of the positive numbers 2^k-18717249721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (18717249791) by changing a digit.

It is a polite number, since it can be written in 2 ways as a sum of consecutive naturals, for example, 68406 + ... + 205216.

Almost surely, 2^18717249721 is an apocalyptic number.

18717249721 is the 136811-st square number.

18717249721 is the 68406-th centered octagonal number.

It is an amenable number.

18717249721 is a deficient number, since it is larger than the sum of its proper divisors (136812).

18717249721 is an frugal number, since it uses more digits than its factorization.

18717249721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 273622 (or 136811 counting only the distinct ones).

The product of its digits is 395136, while the sum is 49.

Multiplying 18717249721 by its sum of digits (49), we get a square (917145236329 = 957677²).

The spelling of 18717249721 in words is "eighteen billion, seven hundred seventeen million, two hundred forty-nine thousand, seven hundred twenty-one".